This- switches the INT3 handler to run on an IST stack (to cope with breakpoints set by a kernel debugger on places where the kernel's %gs base hasn't been set up, yet); the IST stack used is shared with the INT1 handler's- allows nesting of INT1/INT3 handlers so that one can, with a kernel debugger, debug (at least) the user-mode portions of the INT1/INT3 handling; the nesting isn't actively enabled here since a kernel- debugger-free kernel doesn't need it